1. Field of the Invention
This invention relates to a picture taking lens capable of preventing image deterioration caused by accidental inclination or so-called a camera shake when a picture is being taken, and more particularly to a compensating means particularly suited to a zoom lens which has a variable focal length.
2. Description of the Prior Art
When taking a picture from a moving automobile or airplane, or when a long focus lens is used, a significant image deterioration caused by a camera shake tends to occur. Heretofore, many proposals have been made to provide means for compensating for image blur caused by camera shake.
As a simple means for compensating for image blur, in principle, in a lens system which comprises a plurality of lens groups 1 and 2 as shown in FIG. 27(A), there can be contemplated one compensation system, in which a lens group nearest to an object (hereinafter simply referred to as the "object side group") 1 in the lens system is decentered from the optical axis of the lens system in order to obtain a still image on an image surface 3.
As shown in FIG. 27(B), in the case that the optical axis of the lens system is inclined from an original position indicated by a broken line to a position indicated by a one-dotted chain line, image blur can be compensated by decentering the object side group 1, which is supposed to be in a position indicated by the broken line if the lens group 1 was decentered together with the lens system, to a position indicated by a solid line.
In this case, it is apparent that the object side group 1 may simply be moved in a direction perpendicular to the optical axis by an amount equal to that of the blur of an image which is formed by the group 1. Given that the focal length of the object side group 1 is f.sub.1 and an angle of inclination of the whole lens which causes camera shake is .omega..sub.t, the moving amount .DELTA.Y can be expressed as follows; EQU .DELTA.Y=f.sub.1 .multidot..omega..sub.t
However, the object side group of an observation optical system usually has a large effective aperture, and this is more significant when the lens is a telephoto lens having a long focal length. Therefore, the load on a mechanism for moving the lens groups becomes large, and thus, the system eventually becomes large.
In view of the foregoing, in a lens system comprising a plurality of lens groups, there have been made several proposals, as seen, for example, in Japanese Patent Early Laid-open Publication No. Sho 62-203119 and Japanese Patent Early Laid-open Publication No. Sho 62-47011, in which a lens group nearer to an image, i.e., an image side group, is decentered to obtain a still image. That is, as shown in FIG. 27(C), in the case that the optical axis of a lens system is inclined from an original position indicated by a broken line, to a position indicated by a one-dotted chain line, image blur can be compensated by decentering the image side group 2, which is supposed to be in a position indicated by the broken line if the group 2 was decentered together with the lens system, to a position indicated by the solid line.
In a constitution as mentioned above, given that a composite focal length of a lens group nearer to the object than a lens group which is moved for compensation is f.sub.1, an angle of inclination caused by camera shake is .omega..sub.t, and a magnification of the image blur compensating lens group is m.sub.c, the decentering of an image at the center of a screen can be reduced to zero by setting the moving amount .DELTA.Y of the image blur compensating lens group in the direction within a plane perpendicular to the optical axis for compensating for image blur as follows; EQU .DELTA.Y=f.sub.1 .multidot.M.multidot..omega..sub.t EQU wherein, EQU M=m.sub.c /(1-m.sub.c)
However, if a lens group, which is moved for focusing, is served by the image blur compensating lens group as disclosed in Japanese Patent Early Laid-open Publication No. Sho 62-47011, there arise problems such as a complicated mechanism, slow autofocus speed or the like.
Furthermore, in a zoom lens, there arises another problem, i.e., how to arrange the variable power lens group in relation to the image blur compensating group. That is, in the case that the image blur compensating group is placed in a position nearer to the object than the variable power lens, blur compensation is completed by the image blur compensating lens. Therefore, a coefficient of compensation showing the ratio of a moving amount with respect to a blur amount in any focal length to be changed by the zooming is established only in a single way and therefore, there can be obtained an easy control.
However, in the case of the zoom lens, as the lenses nearer to the object than the variable power lens group generally serve as a focusing lens group, the same problems as mentioned above arise.
On the other hand, in the case that the image blur compensating group is arranged in a position nearer to the image than the variable power lens group, the coefficient of compensation becomes, in general, a function of a composite focal length by a lens group nearer to the object than the image blur compensating group. Therefore, a calculation process is required to achieve obtaining a moving amount for compensation in accordance with each focal length.